This invention relates to improved aluminum alloy sheet stock, and more particularly, it relates to a method of providing coated aluminum sheet stock having improved strength and high levels of formability.
Aluminum alloys such as AA5182 have found widespread use as easy-open ends for containers. The sheet from which an end is made must have sufficient formability to make an integral rivet and a fracture-free score as well as sufficient strength to resist buckling caused by internal pressurization of the can. When it is desired to downgauge end stock, such as AA5182, many different problems arise because of constraints already imposed upon the sheet. For example, to downgage end stock sheet requires the use of even higher strength alloys. However, typically with higher strength alloys, formability decreases. Thus, it will be seen that it is highly desirable to provide higher strength alloys with no loss in formability in order that the integrity of the integral rivet and score line on the can end is not compromised.
Examples of compositions suitable for end stock are provided in U.S. Pat. Nos. 3,560,269 and 3,502,448, which are typical of work-hardened 5000 series alloys. Such alloys are known to suffer from progressive room or elevated temperature age softening (reduction in strength) with time. In the past, such alloys have been subjected to thermal softening treatments to lower the mechanical properties to a level where they will be stable over a long period of time. This treatment provides for much the same level of formability regardless of the time between fabricating the sheet and forming the can ends and is referred to as stabilization. The stabilizing process is different from partial annealing which is used to achieve specific strength and ductility properties through thermal softening of the sheet.
To make thinner sheet suitable for beverage can ends, there is a great need for an improved process which will provide for improved strength levels while maintaining a high level of formability and which process will not lower mechanical properties, such as results from stabilization or annealing.